High yield-strength steel for low-temperature services

ABSTRACT

HIGH-YIELD-STRENGTH STEEL FOR LOW-TEMPERATURE SERVICES CONSISTING ESSENTIALLY OF 0.01 TO 0.20% C, 0.10 TO 0.60% SI, 1.00 TO 1.50% MN, 0.10 TO 0.30% CU, 0.10 TO 0.40% CR, 0.020 TO 0.060% SOL AL, 0.008 TO 0.020% N AND THE BALANCE BEING FE AND UNAVOIDABLE IMPURITIES, SAID STEEL HAVING HIGH TENSILE-STRENGTH OF MORE THAN 50 KG./MM.3, HIGHLY YIELD-STRENGTH OF MORE THAN 35 KG./MM.2, EXCELLENT LOW-TEMPERATURE TOUGHNESS AND GOOD WELDABILITY.

United States Patent 3,573,898 HIGH YIELD-STRENGTH STEEL FOR LOW-TEMPERATURE SERVICES Shigeki Murai, Shunichi Shimada, and Nobuhiko Nakao, Yokohama-shi, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan No Drawing. Filed Jan. 31, 1969, Ser. No. 795,705 Claims priority, application Japan, Feb. 5, 1969, 44/ 6,710 Int. Cl. C22c 39/20 U.S. Cl. 75-124 4 Claims ABSTRACT OF THE DISCLOSURE High-yield-strength steel for low-temperature services, consisting essentially of 0.01 to 0.20% C, 0.10 to 0.60% Si, 1.00 to 1.50% Mn, 0.10 to 0.30% Cu, 0.10 to 0.40% Cr, 0.020 to 0.060% Sol Al, 0.008 to 0.020% N and the balance being Fe and unavoidable impurities, said steel having high tensile-strength of more than 50 kg./mm. highly yield-strength of more than 35 kg./mm. excellent low-temperature toughness and good weldability.

The present invention relates to a high yield-strength steel for low temperature services. The invention provides a steel having, at low temperatures, a high tensile strength, a high yield point, good ductility, excellent notch toughness, and good weldability.

Heretofore, steels such as aluminum killed steels of Si- Mn system have been used for industrial purposes. These steels are not always suitable for use in a field requiring high tensile strength and low-temperature toughness Since they lack tensile strength and their yield point is relatively low.

An object of the present invention is to provide economical steel having an excellent low-temperature toughness, which is based on coexistence of Mn and Aln, without the addition of expensive elements such as Ni and the like.

Another object of the present invention is to provide economical steel having a far higher toughness and strength at low temperature services than ordinary Alkilled steels of Si-Mn system.

3,573,898 Patented Apr. 6, 1971 The amount of each element in steels of the present invention is restricted to the stated ranges for the following reasons. Less than 0.10% C lowers the tensile strength and more than 0.20% C causes the toughness at a low temperature to decrease and is harmful to the weldability. 0.10 to 0.60% Si and 1.00 to 1.50% Mn are necessary because they act as effective deoixidzers and also provide in expensive steels having a higher tensile strength and yield point; the relatively large amount of Mn (1.00 to 1.50%) is added because good toughness at a low temperature is obtained. The upper limits of Si and Mn content are 0.60% and 1.50% respectively because at a limit exceeding these, the low temperature toughness of steel deteriorates. The lower limits of Si and Mn content are set at 0.10% and 1.00% respectively as less than these amounts will not yield the/expected effects mentioned above.

Cr and Cu are added to increase the tensile strength and yield point, the optimum range added being 0.10 to 0.40% Cr and 0.10 to 0.30% 'Cu.

Al is added to stabilize solution N in steels as aluminum nitride by heat-treatment such as quenching, tempering or nomalizing and to control crystal grain-size of steel. The above Sol Al, which is 0.020 to 0.060% content, is very effective to improve low-temperature toughness owing to the interactions of the above-mentioned Mn and the aluminum-nitride made to precipitate according to a large amount of 0.008 to 0.020% N added.

The lower limit of S01 Al content is 0.020% because with less than this amount it is impossible to completely stabilize the solution N in steel. Such N content is restricted within the above range, which is 0.008 to 0.020% being equivalent to said Sol Al content, which is 0.020 to 0.060%.

Using the above described ranges of chemical composition, we have succeeded in obtaining steels having a high tensile strength, yield point, and excellent toughness at a low temperature services.

The invention is illustrated by the following example.

EXAMPLE Table I illustrates a steel having a chemical composition embodying the present invention.

TABLE I Plate thickness 0 Si Mn P S Cu Cr Sol Al Total N A, 25 mln 0. 18 0. 38 1. 31 0. 0017 0. 0012 0. 12 0. 12 0. 048 0. 0124 B, 32 nlm 0. 19 0. 39 1. 31 0. 0017 0. 0012 0. 12 0. 12 0. 048 0. 0124 Additional objects of this invention will become apparent by the following descriptions, references being made to the examples.

According to the present invention there is provided a high-strength steel for low temperature services, consisting essentially of the following elements in the stated percentages by weight:

and the balance being Fe and unavoidable impurities. Steels of this invention have a much higher yield point and tensile strength than the aforementioned conventional aluminum killed steels of Si-Mn system, and also low temperature toughness same as or higher than that of these steels.

Mechanical properties of the steel shown in Table I after normalizing are shown in Table II.

TABLE II Tensile strength test V-notch Charpy impact test value Plate Yield Tensile Elongathickness point strength tion -60 C. 0 0.

mm.) (kg/mmfl) (kg/mm!) (percent) (kgr /cmfl) The mechanical properties of steel embodying this invention heat treated by water-quenching at 920 C., and then by tempering at 660 C., which is at the rate of 70 minutes per every 25 mm. plate thickness are shown in Table III.

As is clearly shown in the above example, steels of the invention maintain a high tensile strength of approximately 50 kg./rnm. when normalized, While their lowtemperature toughness as low as 60 C. continues to be high.

It is understood that when heat treated by quenching and tempering, the yield point, tensile strength and low temperature toughness of steel is further increased.

In comparison with conventional aluminum killed steels of Si-Mn system, used for similar purposes and of the same cost, the present invention provides steels with much higher tensile strength and yield point (about 60 kg./mm. and 48 kg./mm. respectively) than are obtainable with the conventional steels. Their low-temperature toughness may be the same as or even better than the said conventional steels. Therefore, the steels of the invention are best applied in such fields as marine constructions and the like for cold areas or in the frigid zones where both high tensile strength and low-temperature toughness are required. They show excellent results enabling them to be serviceable as high strength steel.

What we claim is:

1. A high yield-strength steel for low-temperature services consisting essentially of the following elements in the stated percentages by weight:

Percent Sol Al 0020-0060 and the balance being Fe and unavoidable impurities.

2. A low-temperature tough steel consisting essentially of the following elements in the stated percentages by weight:

Percent Si 0.10-0.60 Mn 1.00-1.50

Cr 0.10-0.40 Sol Al 0020-0060 and the balance being Fe and unavoidable impurities.

3. A steel defined by claim 1 containing in percentages by weight:

Percent S 0.0012 Cu 0 .12

Cr 0.12 Sol Al 0.048

4. A steel defined by claim 1 containing in percentages by weight:

HYLAND B-IZOT, Primary Examiner US. Cl. XiR. -125, 126

UNITED STATES PATENT OFFICE 5B CERTIFICATE OF CORRECTION Patent No. 3,573,898 Dated April 6, 1971 Inventor) SHIGEKI MURAI et 'al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 8 priority date incorrectly identified "February 5, 1969" should be corrected to read "February 5, 1968", as indicated in Declaration filed with application.

Column 1, line 14 after "High" delete hyphen and line 20 "5O kg/mm. should be corrected to read "50 kg/mm. all as indicated in ABSTRACT filed with application.

Column l, line 38 "Aln" should be corrected to read "Al as indicated at page 1, line 11 of specificatio Signed and sealed this 21st day of December 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,J'R. ROBERT GOTISGHALK Attesting Officer Acting Commissioner of Pater 

